As devices such as robotic vehicles get smaller (i.e., iRobot®'s Packbot®) in settings where autonomous navigation requirements are lessened, the operational requirements for ranging sensors such as LADAR sensing systems are reduced but not all together eliminated. In these and other applications, fewer LADAR sensors per vehicle are required but such sensors also must be smaller, more efficient and lower power. Further, a full 360-degree field of regard is not typically required, i.e., in certain military applications, a 60-degree (horizontal)×30-degree vertical field of regard with a range capability of about 10 meters is all that is required.
In view of the above, industry is researching and developing a number of LADAR approaches such as solid-state LADAR scanning and staring methods and devices that miniaturize the scanning elements of the LADAR ranging devices.
LADAR devices are basically photon time-of-flight (“TOF”) measurement devices that are used for range measurement of the surface of an object. Time-of-flight devices utilize a number of approaches to measure the time between the transmission of a signal (such as a laser pulse or beam), and its return (i.e., reflection or echo). Examples of such approaches include pulsed-modulation LADAR, pseudo-noise modulation LADAR and continuous-wave modulation LADAR. Continuous-wave modulation LADAR includes, for instance, such categories as AM chirp, AM homodyne, AM heterodyne, FM chirp and AM phase-coding methods among others.
Selected examples of miniaturized LADAR sensor systems in development are also discussed in “3D Time-of-Flight Distance Measurement with Custom Solid-State Image Sensors in CMOS-CCD Technology” (R. Lange, Ph.D. Dissertation, Dept. of Electrical Engineering and Computer Science, University of Siegen, (2000)), the entirety of which is incorporated herein by reference.
As a further example of existing miniaturized LADAR devices, Mesa Imaging AG (www.mesa-imaging.ch) has eliminated mechanical scanning altogether and provides a solid-state “staring” mosaic LADAR device (e.g., the Swiss Ranger 4000) with acceptable performance and range for certain military applications.
Unfortunately, the performance of this and other staring and scanning LADAR systems is severely degraded in operation outdoors, due in significant part to high solar background infrared radiation (IR) at ground level which is the result of the solar radiation spectrum incident upon the surface of the Earth.
The device of the Swiss Ranger staring mosaic LADAR operates in the very-near IR region and projects a frequency modulated sine wave of laser energy with an 850-nm wavelength upon a scene of interest (i.e., an illumination beam). The modulation frequency is predetermined by the design of the system.
The receiving CCD imager of the Swiss Ranger then captures the return laser pulse along with unwanted solar background IR energy. The phase difference between the transmitted sine wave and the reflected and received sine wave is computed using a predetermined number of return signal samples which are in turn used to calculate the proportional to the time-of-flight or range to the surface of an object.
The high solar background IR is unavoidably received in outdoor operation and not only degrades the signal-to-noise ratio of the system, but also severely degrades the system's modulation contrast.
By illuminating the scene with 850-nm laser energy, the prior art systems are undesirably bounded by two issues: 1) the system cannot increase the transmission power without becoming non-eye safe, and, 2) the system cannot reduce the received background IR energy by narrowing the receiver spectral filter significantly without having to compensate for laser line drift due to temperature variations or for spectral filter bandpass drift due to various illumination incident angles.
The invention herein is beneficially utilized in both staring LADAR and scanning LADAR devices, including any of the TOF LADAR approaches above (e.g., pulsed-modulation LADAR, pseudo-noise modulation LADAR and continuous-wave modulation approaches) and avoids the above deficiencies in the prior art.
For instance, the invention provides a staring mosaic approach as utilized by the Swiss Ranger but illuminates a scene using laser energy at wavelengths approximately that of one or more of the atmospheric transmission holes, preferably at 1.39 microns. This beneficially results in the reduction of total transmitted solar irradiance seen by the receiver by nearly three orders of magnitude while still permitting LADAR transmission at 20 meters of greater than 65%.
A further benefit of the method of the invention is that the illumination of the scene with laser wavelengths at 1.39 microns is operating in a more eye-safe operating region, thus allowing up to a full order of magnitude increase in laser illumination power where necessary.